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Date: Mon, 12 Feb 2024 15:29:00 +0100
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Subject: Re: [PATCH v5 03/25] mm: Make pte_next_pfn() a wrapper around
 pte_advance_pfn()
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To: Ryan Roberts <ryan.roberts@arm.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>,
 James Morse <james.morse@arm.com>, Andrey Ryabinin <ryabinin.a.a@gmail.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Mark Rutland <mark.rutland@arm.com>,
 Kefeng Wang <wangkefeng.wang@huawei.com>, John Hubbard
 <jhubbard@nvidia.com>, Zi Yan <ziy@nvidia.com>,
 Barry Song <21cnbao@gmail.com>, Alistair Popple <apopple@nvidia.com>,
 Yang Shi <shy828301@gmail.com>, Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-arm-kernel@lists.infradead.org, x86@kernel.org,
 linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20240202080756.1453939-1-ryan.roberts@arm.com>
 <20240202080756.1453939-4-ryan.roberts@arm.com>
 <c269c2e0-afca-4ff0-8ffd-9049fb52c905@redhat.com>
 <cdf4e537-5b0d-45ba-8094-527127c791a6@arm.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <cdf4e537-5b0d-45ba-8094-527127c791a6@arm.com>
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On 12.02.24 15:10, Ryan Roberts wrote:
> On 12/02/2024 12:14, David Hildenbrand wrote:
>> On 02.02.24 09:07, Ryan Roberts wrote:
>>> The goal is to be able to advance a PTE by an arbitrary number of PFNs.
>>> So introduce a new API that takes a nr param.
>>>
>>> We are going to remove pte_next_pfn() and replace it with
>>> pte_advance_pfn(). As a first step, implement pte_next_pfn() as a
>>> wrapper around pte_advance_pfn() so that we can incrementally switch the
>>> architectures over. Once all arches are moved over, we will change all
>>> the core-mm callers to call pte_advance_pfn() directly and remove the
>>> wrapper.
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>> ---
>>>    include/linux/pgtable.h | 8 +++++++-
>>>    1 file changed, 7 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>> index 5e7eaf8f2b97..815d92dcb96b 100644
>>> --- a/include/linux/pgtable.h
>>> +++ b/include/linux/pgtable.h
>>> @@ -214,9 +214,15 @@ static inline int pmd_dirty(pmd_t pmd)
>>>        #ifndef pte_next_pfn
>>> +#ifndef pte_advance_pfn
>>> +static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
>>> +{
>>> +    return __pte(pte_val(pte) + (nr << PFN_PTE_SHIFT));
>>> +}
>>> +#endif
>>>    static inline pte_t pte_next_pfn(pte_t pte)
>>>    {
>>> -    return __pte(pte_val(pte) + (1UL << PFN_PTE_SHIFT));
>>> +    return pte_advance_pfn(pte, 1);
>>>    }
>>>    #endif
>>>    
>>
>> I do wonder if we simply want to leave pte_next_pfn() around? Especially patch
>> #4, #6 don't really benefit from the change? So are the other set_ptes()
>> implementations.
>>
>> That is, only convert all pte_next_pfn()->pte_advance_pfn(), and leave a
>> pte_next_pfn() macro in place.
>>
>> Any downsides to that?
> 
> The downside is just having multiple functions that effectively do the same
> thing. Personally I think its cleaner and easier to understand the code with
> just one generic function which we pass 1 to it where we only want to advance by
> 1. In the end, there are only a couple of places where pte_advance_pfn(1) is
> used, so doesn't really seem valuable to me to maintain a specialization.

Well, not really functions, just a macro. Like we have set_pte_at() 
translating to set_ptes().

Arguably, we have more callers of set_pte_at().

"Easier to understand", I don't know. :)

> 
> Unless you feel strongly that we need to keep pte_next_pfn() then I'd prefer to
> leave it as I've done in this series.

Well, it makes you patch set shorter and there is less code churn.

So personally, I'd just leave pte_next_pfn() in there. But whatever you 
prefer, not the end of the world.

-- 
Cheers,

David / dhildenb